The term and concept of "allergy" was coined by a Viennese pediatrician named Clemens von Pirquet in 1906. He observed that the symptoms of some of his patients might have been a response to outside allergens such as dust, pollen, or certain foods. For a long time all hypersensitivities were thought to stem from the improper action of inflammatory immunoglobulin class IgE, however it soon became clear that several different mechanisms utilizing different effector molecules were responsible for the myriad of disorders previously classified as "allergies". A new four-class (now five) classification scheme was designed by P. G. H. Gell and R. R. A. Coombs. Allergy has since been kept as the name for Type I Hypersensitivity, characterised by classical IgE mediation of effects.

Definition of Allergies
Allergies are abnormal reactions to ordinarily harmless substances. The sensitizing substances, called allergens, may be inhaled, swallowed, or come into contact with the skin.
Allergens that most frequently cause problems are: pollens, mold spores, house dust mites, animal danders, foods, insect bites or stings, plants, insect spores, latex rubber, viruses, bacteria, medications and environmental conditions (such as cold temperatures). Allergic reactions can occur in one area, such as sneezing or a skin rash or sneezing, or may include more than one symptom.

Description of Allergies
Normally, the body learns to defend itself through experience - by encountering, battling and remembering one enemy after another. For decades, medical science has taken advantage of this ability by using vaccination to create immunity - the immunologic "memory" of a disease. Allergic reactions occur after the immune system mistakenly learns to recognize innocent foreign substances (allergens) as potentially harmful.
The following story illustrates how an allergy can develop:

Over a field of ragweed plants floats an invisible cloud of pollen grains, soon carried by the wind into a nearby town. The pollen is inhaled by a child whose body has never been exposed to this substance before.

Because of some genetic predisposing factor, this child's immune system overreacts and produces large numbers of IgE antibodies, all specially designed to respond to ragweed pollen. Several of the antibodies attach themselves to cells in the child's nasal passages and upper respiratory tract.

These cells (known as mast cells) contain strong chemicals called mediators, the best-known of which is histamine.

Later, when the child inhales the same kind of pollen again, proteins from the pollen bind in a lock-and-key fashion to the specially designed antibodies on the surface of the mast cells. This sets off an explosion of sorts, as the mediators burst from inside the mast cells, destroying the pollen and also damaging surrounding tissues. The results are sneezing, a stuffy head, sniffling, stuffed-up head and red, watery eyes - well-known hallmarks of allergies.

Causes and Risk Factors of Allergies
The fundamental cause of allergy is still not known. The problem has a tendency to run in families. An allergic individual is more likely to have relatives who are allergic than would be expected on the basis of chance, but non-hereditary factors apparently play a part as well. Evidence of this is the fact that infants who are breast-fed are less likely to develop allergies than bottle-fed babies.

The reason an individual becomes sensitive to some substances and not to others remains a mystery.

Individuals can be affected by a variety of allergic diseases. The most common allergic diseases are allergic rhinitis, non-allergic rhinitis, asthma, allergic dermatitis, contact dermatitis and urticaria. (Allergic rhinitis is the most common of the allergic diseases and the main focus of this health profile. More comprehensive information about the other conditions may be found under that particular term.)

Allergic rhinitis is a general term used to apply to anyone who has allergy-based symptoms. Allergic rhinitis can be a seasonal problem (commonly known as "hay fever" or pollen allergy) or a year-round problem (commonly known as perennial allergic rhinitis). Hay fever or seasonal allergic rhinitis is caused by allergy to pollens of trees, grasses, weeds or mold spores. Perennial allergic rhinitis is caused by house dust, animal danders, mold and some foods.

Asthma is caused by intrinsic and extrinsic (inhaled) factors. Intrinsic factors are pollens, dust, dust mites, animal fur, animal dander or feathers. Extrinsic factors are respiratory infections; a cough, cold or bronchitis; exercise and tobacco smoke or other air pollutants, and can be caused by an allergy to a particular food or medication.

Eczema, also known as allergic dermatitis or atopic dermatitis, can be caused by foods or other allergens.